Damascene processes for forming integrated circuit metalization layers employ electroplated copper lines formed in vias and trenches of supporting dielectric layers. Copper atoms are rather mobile and can easily diffuse or migrate into the supporting dielectric and thereby reduce its resistance. To address this problem, damascene processes employ thin diffusion barrier layers on the entire exposed surface of the dielectric. These barrier layers are made from a material that effectively blocks transport of copper atoms into the adjacent dielectric. Typically, the barrier layers comprise slightly conductive titanium nitride or tantalum nitride at a thickness of approximately 10 to 100 angstroms.
The diffusion barrier materials are not sufficiently conductive to support direct electroplating of copper from solution. So damascene processes first deposit a thin copper seed layer over the entire exposed diffusion barrier layer. This layer is typically formed by physical vapor deposition or electroless deposition and has a thickness of approximately 50 to 1500 angstroms. Onto the seed layer, the damascene process deposits a bulk layer of copper by electroplating. Electroplating fills all vias and trenches and continues until copper covers all exposed dielectric. Finally, the excess copper is removed by chemical mechanical polishing to provide a planar surface of exposed copper lines encased by dielectric and diffusion barriers.
One problem encountered in the above process involves poor adhesion of the copper seed layer (and copper in general) to the underlying diffusion barrier layer. It turns out that copper does not adhere well to the diffusion barrier materials. In fact, copper does not adhere well to a wide array of materials employed in integrated circuits. One notable exception is tantalum. So, possibly the damascene process flow could employ a tantalum adhesion layer interposed between the diffusion barrier layer and the copper seed layer.
Known processes for depositing thin tantalum layers on substrates include chemical vapor deposition (CVD) employing organometallic precursors. The deposition reactions are often endothermic (or have a positive ΔG). As a consequence, process must provide energy to drive the deposition reaction. This involves heating the wafer, including the previously deposited copper within the wafer. At approximately 450 degrees C., copper's material properties begin to transform so that it becomes more mobile. Unfortunately, there are currently no thermal CVD processes available for depositing tantalum films at temperatures below 450 degrees C.
A lower-temperature alternative to using traditional thermal CVD is atomic layer deposition (ALD), also known as atomic layer epitaxy (ALE) or atomic layer chemical vapor deposition (ALCVD). Unlike CVD, ALD relies on a self-limiting, saturated, surface, growth mechanism in which the reactants are introduced alternately over the substrate surface, separated by inert gas purging. Deposition temperature is selected such that only a saturated layer of precursor is adsorbed on the substrate surface and does not depend as much on reactant flow as the relatively higher temperatures that CVD demands.
Equally important as the technique to depositing tantalum is the choice of reactant precursors. In ALD, the precursor must readily adsorb in a self-limiting mode, and once adsorbed must readily react with the co-reactant to form the desired film. Thus, many useful CVD precursors are not viable as ALD precursors, and it is not trivial to select a precursor for the ALD method. In a metal ALD process, it is especially challenging to find a metal precursor that is stable against decomposition, adsorbs evenly on the surface and can also be easily reduced.